Abstract:The continuing rise in turbine entry temperatures has necessitated the development of ever-more advanced cooling techniques. Effusion cooling is an example of such a system and is characterised by a high density of film cooling holes that operate at low blowing ratios, thereby achieving high overall cooling effectiveness. This paper presents both an experimental and computational investigation into the cooling performance of effusion systems. Two flat-plate geometries (with primary hole pitches of 3.0D and 5.75D) are experimentally investigated via a pressure sensitive paint technique yielding high resolution film effectiveness distributions via heat-mass transfer analogy. A computational fluid dynamics (CFD) scalar tracking method was used to model the setup computationally with the results comparing favourably to those obtained from the experiments. The CFD domain was modified to assess the cooling performance from a single film hole ejection. A superposition method was developed and applied to the resulting two-dimensional film effectiveness distribution that quickly yielded data for an array of closely-packed holes, allowing a rapid assessment of a multi-hole effusion type setup. The method produced satisfactory results at higher pitches, but at lower pitches, high levels of jet interactions reduced the performance of the superposition method.

The trailing edge of the high pressure turbine blade presents significant challenges to the turbine cooling engineer. A novel cooling design using cross corrugated slots for the trailing edge has been proposed. This geometry allows blade designers to finely tune pressure loss and consequently coolant flow through the slot, but potentially results in poor film cooling performance downstream of the slot exit, an effect that could be mitigated with exit shaping. The current study is focused on comparing film cooling effectiveness on the cutback surface and lands with a plain rectangular slot under the same conditions. A set of nine cross corrugated internal slot geometries has been investigated in a large scale model of the trailing edge pressure side ejection slot exit. Four geometries used a 90° included angle with variations to the channel alignment at slot exit. Four used a 120° included angle, with the same variations to the exit alignment. The final geometry used a 90° included angle with exit shaping. Pressure sensitive paint was used to measure adiabatic film cooling effectiveness at five blowing ratios ranging from 0.6 to 1.4 in increments of 0.2. High resolution 2D distributions of film cooling effectiveness both on the cutback surface and the top of the lands were recorded. It was found that unmodified cross corrugated slots do result in poor film effectiveness on the cutback surface compared to a plain rectangular slot. However, land cooling is slightly improved, and applying exit shaping to the cross corrugated slot results in effectiveness levels at the trailing edge on par with or even superior to the rectangular slot at blowing ratios of 0.8 or below. Therefore, in this respect, the novel cross corrugated slot design proposed is a viable candidate for blade design, provided exit shaping is used and low blowing ratios are expected.

The effect of film cooling on a high pressure turbine blade with an open squealer tip has been examined in a high speed linear cascade. The cascade operates at engine realistic Mach and Reynolds numbers, producing transonic flow conditions over the blade tip. Tests have been performed on two uncooled tip geometries with differing pressure side rim edge radii, and a cooled tip matching one of the uncooled cases. The pressure sensitive paint technique has been used to measure adiabatic film cooling effectiveness on the blade tip at a range of tip gaps and coolant mass flow rates. Complementary tip heat transfer coefficients (HTC) have been measured using transient infrared thermography, and the effects of the coolant film on the tip heat transfer and engine heat flux examined. The uncooled data show that the tip heat transfer coefficient distribution is governed by the nature of flow reattachments and impingements. The squealer tip can be broken down into three regions, each exhibiting a distinct response to a change in the tip gap, depending on the local behaviour of the overtip leakage flow. The edge radius of the pressure side rim causes the overtip leakage flow to change dramatically at low clearance. Complementary CFD shows that the addition of casing motion causes no further change on the pressure side rim. Injected coolant interacts with the overtip leakage flow, which can locally enhance the tip heat transfer coefficient compared to the uncooled tip. The film effectiveness is dependent on both the coolant mass flow rate and tip clearance. At increased coolant mass flow, areas of high film effectiveness on the pressure side rim coincide strongly with a net heat flux reduction and in the subsonic tip region with low heat transfer coefficient.

The trailing edge of the high pressure turbine blade and vane presents significant challenges to the turbine cooling engineer. The current research has focused specifically on the effect of cutback surface protuberance, or "land", shapes on film cooling effectiveness. A set of six different land geometries has been investigated in a large scale model of the trailing edge pressure side ejection slot exit. Slot height and width and lip height was maintained. Pressure sensitive paint was used to measure adiabatic film cooling effectiveness at five blowing ratios ranging from 0.6 to 1.4 in increments of 0.2. High-resolution full surface distributions of film cooling effectiveness both on the cutback surface and the top of the lands were recorded. It was found that tapering the lands did not significantly increase effectiveness on the lands and slightly reduced effectiveness near the lands. Using a diffuser shape improved average effectiveness greatly and gave the best overall performance up to the end of the lands except at the lowest blowing ratio of 0.6, where having no lands was slightly better.

The continuing rise in turbine entry temperatures has necessitated the development of evermore advanced cooling techniques. Effusion cooling, which is characterised by a high density of film holes operating at low blowing ratios, represents one possible mechanism for achieving high overall cooling effectiveness. This paper presents an experimental investigation performed on flat-plate, effusion-type cooling geometries (with primary hole pitches of 3.0D and 5.75D) using pressure sensitive paint to yield high-resolution film effectiveness distributions using the heat/mass transfer analogy. CFD was used to model the setup computationally, with results comparing favourably to the experiments. The CFD domain was then altered to model a single hole. A superposition method was developed and applied to the two dimensional film effectiveness distribution, yielding data for an array of closely-packed holes. The method produced satisfactory results at higher pitches, but at lower pitches, high levels of jet interactions reduced the performance of the superposition method.